1. Field of the Invention
This invention relates to luminescent compositions. More particularly it relates to manganese-activated alkaline earth tungstate where the atomic ratio of the alkaline earth metal to tungsten is from about 3:1 to about 3.4:1.
2. Prior Art
Lead-activated calcium tungstate, wherein the ratio of Ca to W is about 1, is well known. U.S. Pat. Nos. 2,617,773 and 3,790,494 describe these luminescent materials and modifications thereof. Calcium tungstate at 1:1 ratio without an activator is a well-known X-ray phosphor. Strontium and barium tungstate having a Sr:W or a Ba:W of about 1 have also known blue-emitting phosphors.
While it is known that alkaline earth tungstate compounds having a 3:1 alkaline earth to tungsten ratio exist, heretofore there has been no mention of their capability as luminescence materials.
It is believed that a new host material having an alkaline earth metal to tungsten ratio of from about 3:1 to about 3.4:1 when activated by manganese and excited by long wave ultraviolet light that yields a deep red emission is an advancement in the art.